1. Field
The invention relates to a method of bending glazing units by which the glass is sucked by openings passing through a solid concave mold to take the form of said concave mold. The method according to the invention is more particularly suited to a rapid industrial production process, leading to glazing units free of optical defects.
The invention relates most particularly to the simultaneous bending of superposed sheets of glass (usually two superposed sheets of glass) that are required to be assembled later into a laminated glazing unit, particularly of the type of sheets intended to serve as windshields of motor vehicles. The sheets are assembled in a manner known to those skilled in the art by inserting between the sheets of glass a layer of polymer, usually of the polyvinylbutyral type.
2. Description of the Related Art
EP0363097 teaches the suction of a single sheet through a bottom concave mold. After bending, the sheet is separated from the bottom mold by a frame surrounding the concave form and rising to carry the sheet with it. To be able to carry the sheet upward, the latter must protrude beyond all the edges of the concave mold. Such a protrusion is not satisfactory for bending the edges, which are not always precisely formed during the bending. The result of this is a risk of bending defects at the edges of the sheet. Furthermore, this document gives no information relating to the bending of several superposed sheets.
U.S. Pat. No. 3,778,244 teaches a top form fitted with a suction skirt placed above a bottom mold furnished with suction orifices through its solid full surface. These two bending forms work one after the other without carrying out any pressing of the glass, since, when a sheet has been partially bent against the top form, this sheet is allowed simply to fall on the bottom form (col 5 lines 3-6).
WO02064519 teaches the bending of superposed sheets through a bottom concave mold. A top oddside mold may squeeze the edges of the two sheets together at the time of suction by the concave form. Here also, it is a frame surrounding the bottom form which, when rising, picks up the sheets after bending. Consequently, the sheets must also protrude from the bottom form. Furthermore, even though this method represents notable progress, the desire is to be able to accelerate it in the context of an industrial usage. To be able to accelerate it, it is possible to work on the value of the vacuum used during the suction. The applicant has however observed that it was appropriate not to push the vacuum beyond a vacuum pressure of approximately 100 mbar (the difference between atmospheric pressure and the pressure in the bottom mold), because that could lead to optical defects visible to the naked eye, of the distorted vision type. It seems that this defect originates in fact from a clearance being created between the various sheets during the suction. This clearance may even reach 1 mm in the central region. The optical distortions that result therefrom may reach 90 mdpt (millidioptres).